Split card design into front+back, add sound effects & background music

Card design:
- Separate cardFront + cardBack (each own/equip independently)
- Fronts: classic (free), ivory/rosegold (buy), parchment/mint (earned)
- Backs: classic (free), sapphire/emerald (buy), ruby/royal (earned)
- PlayingCard `front` prop; table applies front to all faces, back to opponents
- Profile has front + back pickers; shop has both sections

Audio:
- Web Audio synth engine (no asset files): SFX for card/deal/trump/trick,
  win/lose, message, notify, award, levelup, purchase, kot + ambient music
- Toggles in profile (Audio) + mute button in game HUD; prefs persisted
- Wired across game-store, rewards, daily, shop, chat

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/sound-store.ts

@@ -0,0 +1,26 @@
+"use client";
+
+import { create } from "zustand";
+import { sound } from "./sound";
+
+interface SoundStore {
+  sfx: boolean;
+  music: boolean;
+  toggleSfx: () => void;
+  toggleMusic: () => void;
+}
+
+export const useSoundStore = create<SoundStore>((set, get) => ({
+  sfx: sound.sfxEnabled,
+  music: sound.musicEnabled,
+  toggleSfx: () => {
+    const v = !get().sfx;
+    sound.setSfxEnabled(v);
+    set({ sfx: v });
+  },
+  toggleMusic: () => {
+    const v = !get().music;
+    sound.setMusicEnabled(v);
+    set({ music: v });
+  },
+}));